Ion heating associated with magnetic reconnection is observed in laboratory experiments, the magnetosphere, and in solar flares. In the MST reversed field pinch laboratory plasma, the ions can attain temperatures >1 keV during rapid magnetic reconnection events, well in excess of the electron temperature. This is a clear indication of non-collisional ion heating. The energy reservoir for heating is the mean magnetic field that confines the plasma, but the process by which the energy conversion occurs is not known. New measurements have identified that the energy conversion efficiency depends on the mass of the ions. The fraction of the energy loss in the mean magnetic field that appears as ion thermal energy scales as ΔEtherm / ΔEmag ~ Mi0.5, shown in the figure below. Similar mass dependence is observed in the solar corona and wind. This is an important new clue that should help discriminate possible mechanisms, including viscous dissipation of plasma flows, resonant cyclotron heating in a nonlinear turbulent cascade, and stochastic heating in a turbulent electric field. This work was presented as an invited talk at the 2009 APS April Meeting, and will also be an invited talk at the 2009 APS Division of Plasma Physics Meeting.